The gripping force F of a tire can be given by the product of the frictional coefficient .mu. between the tire and the road surface and the vertical contact load W acting on the tire contact surface (F=.mu.W). In other words, the tire contact load which plays an important role in the handling of the vehicle is proportional to the magnitude of the tire contact load for a given road condition.
In a known active wheel suspension system, a linear actuator which can be actively extended and retracted is typically installed between the vehicle body and each road wheel so that the distribution of tire contact load may be distributed to the four different road wheels according to a prescribed control mode. For instance, U.S. Pat. No. 4,625,993 issued Dec. 2, 1986 to Williams et al. discloses an active wheel suspension system which controls the stroke of the hydraulic actuator provided with each road wheel so that the attitude of the vehicle body may be properly controlled when the vehicle is travelling. When the vehicle is travelling straight ahead, the tires are made to follow the irregular contour of the road surface so that the gravitational center of the sprung mass may be controlled or may stay at a relatively fixed height. When the vehicle is accelerating or decelerating, the load distribution between the front axle and the rear axle is appropriately changed so that the pitching movement of the vehicle may be controlled. When the vehicle is turning a curve, the load distribution between the right and left wheels is appropriately changed so that the rolling movement of the vehicle may be controlled.
According to such a conventional active wheel suspension system, the weight of the vehicle body was simply distributed between the different road wheels, and the sum of the contact pressures of the four road wheels was essentially constant. Therefore, such an active wheel suspension system was not able to increase the overall traction or braking force of the vehicle.
It is known that the maximum gripping force which optimizes the braking force and the traction force can be achieved when the slip ratio of the wheel is at a certain value. The slip ratio is given as a ratio of the difference between the circumferential speed of the tire and the vehicle speed to the vehicle speed. The ABS (antilock brake system) is based on this concept, and is now widely used in motor vehicles. However, the ABS system is not able to change the gripping force of the tire, and typically intermittently release the brake to avoid excessive slipping. In other words, the capability of the ABS system to reduce the braking distance of the vehicle is limited by the given traction force. It is therefore desirable if the gripping force itself can be increased in view of further reducing the braking distance.
The road gripping force of a tire is important also when accelerating a vehicle. When a vehicle is excessively accelerated for a given road condition, the tires start slipping, and not only is a desired acceleration prevented from being achieved, but also the lateral stability of the vehicle may be lost. By noting this problem, it has been proposed to control the traction force of each driven wheel so that the slip ratio of the wheel may be kept within a limit, and a maximum available traction may be obtained at all times. The traction control system is designed to carry out such a control action. However, the conventional traction control system is not able to increase the magnitude of the available traction, and simply reduces the torque transmitted to the wheels so as to prevent the slip ratio from exceeding a prescribed limit.
The frictional coefficient of the road surface for the tires may not be even, and the right and left wheels may travel over road surfaces of different frictional coefficients. This may be called as a split .mu. road surface. When a brake is applied to a vehicle travelling over such a road surface, the vehicle tends to swerve from a straight course, and may even go into a spin. To avoid such condition from occurring, an ABS system normally controls the braking force of each wheel so that the braking force of each of the wheels would not exceed that of the wheel experiencing the smallest frictional coefficient (see Japanese patent laid open publication No. 2-220958). This is beneficial in maintaining the lateral stability of the vehicle, but increases the braking distance of the vehicle. The same problem arises when an attempt is made to accelerate a vehicle traveling over a split .mu. road surface.